The present invention relates to a method and apparatus for magnetic resonance imaging for obtaining tomograms of desired portions of an object to be examined using nuclear magnetic resonance (abbreviated as NMR hereinafter). In particular, it relates to a magnetic resonance imaging method and apparatus capable of obtaining a desired range of images of excellent quality in minimal time to enable visualization of movement in the vascular system.
A magnetic resonance imaging apparatus (abbreviated as MRI apparatus hereinafter) utilizes NMR to measure density distribution and relaxation time etc. of atomic nuclei in a desired portion of an object to be examined utilizing NMR and,displays images of desired slices of the object produced from the measured data. Conventional MRI apparatuses have a blood flow imaging function called MR angiography (abbreviated as MRA hereinafter). This function includes a method using a contrast agent and a method using no contrast agent.
In a common method using a contrast agent, a gradient echo type sequence of short TR (repetition time) is used in combination with a T1-shortening type contrast agent such as Gd-DTPA. The principle of this method will be explained briefly. In the MRI apparatus, when excitation by an RF magnetic field is repeatedly performed at a short interval of several tens of milliseconds for the same area, nuclear spins (sometimes referred to simply as xe2x80x9cspinsxe2x80x9d hereinafter) included in tissues of the area saturate and, consequently, the strength of NMR signals (echo signals) obtained therefrom decreases. On the other hand, blood spins containing a T1 shortening contrast agent are not likely to be saturated by the repeated excitation of a short TR because the blood spins have a shorter T1 than those of surrounding tissues, and generate high-strength signals relative to the surrounding tissues. As a result, blood vessels filled with blood containing a contrast agent can be visualized with high contrast relative to the other tissues. Utilizing this fact, NMR measurement of the region is conducted while the contrast agent remains in the blood, of the region concerned, and the obtained three-dimensional image data are processed to image the blood vessel.
Although the MRA can thus visualize the blood containing a contrast agent as high-strength signals, when small blood vessels are to be imaged, it often cannot provide sufficient contrast difference between the small blood vessels and the surrounding tissues. In order to overcome this problem, as shown in FIG. 16, subtraction operation is often conducted between images obtained before and after administration of the contrast agent to delete the tissue other than the blood vessels. This method is called 3D-MRA-DSA (Digital Subtraction Angiography).
In clinical diagnosis of diseases, not only arterial vessels but venous vessels are required to be imaged. As is well known, in the blood circulation system of a living body, blood from the heart passes through arterial vessels, tissues and venous vessels, and back to the heat via heat-lung heart circulation. Accordingly, when a contrast agent is injected into an elbow vein, the contrast agent mixed with blood goes to the heart first, and then out from the heart to enable visualization of arterial vessels, capillary vessels and venous vessels in this order. This means the contrast MRA measurement should be performed successively over plural time phases in order to image the vessel of interest. This imaging method is called dynamic MRA. Various kinds of contrast MRA are described in detail in xe2x80x9c3D Contrast MR Angiographyxe2x80x9d (2nd edition. Prince MR Grist TM and Debatin JF Springer, P3-P39, 1988).
In the aforementioned contrast MRA measurement techniques, it is necessary for the blood in the vessel of interest to be imaged to have optimum density of the contrast agent at the measurement time, and for the optimum density to be maintained for a certain period of time in order to obtain good image quality. In addition, the contrast agent density should be constant during measurement of low frequency components of the k-space in order to image blood vessels with high contrast.
However, as shown in FIG. 16, the time when the contrast agent density reaches its optimum value differs depending on the site A, B, C even within the same blood vessel. Arrival time difference of the optimum density depends on the blood circulation time of the patient. Accordingly, if the measurement is started too early relative to administration of a contrast agent, the contrast agent will not yet have reached all parts of the blood vessel of interest and, as a result, the blood vessel cannot be imaged. On the other hand, when the start time of the measurement is late, the contrast agent will have already left the region of the blood vessel of interest. As a result, the contrast of the blood vessel becomes low and other unnecessary vessels are imaged. Thus, it has been impossible to image only the blood vessel of interest with high contrast.
Further, if the start of the measurement is mistimed, then when the venous system is imaged successively after imaging of the arterial system, not only a vein image but also an artery image is obtained and, as a result, the two are not distinguishable.
The present invention aims at solving the above-mentioned problems and one object of the present invention is to provide an MRI method capable of imaging the entire blood vessel of interest with high contrast and displaying images useful for diagnosis. Another object of the present invention is to image veins and arteries in a distinguishable manner.
In one aspect, the present invention provides a magnetic resonance imaging method comprising the steps of applying a static magnetic field, gradient magnetic field and RF magnetic field to an object to be examined according to a predetermined pulse sequence, causing nuclear magnetic resonance in nuclear spins within a predetermined region of the object, measuring NMR signals caused by the NMR, and producing and displaying images using the measured signals, wherein a plurality of time-series NMR signal groups are obtained, on an individual image basis, for the same region of the object, one of the plurality of NMR signal groups is defined as standard data, a plurality of subtracted NMR signal groups are produced by performing a subtraction operation between the standard NMR signal group and the other NMR signal groups, the subtracted NMR signal groups are subjected to addition operation and a NMR signal group subtraction obtained by the addition operation is displayed.
In the addition operation, each of the subtracted NMR signal groups is weighted using a weighting coefficient. The weighting coefficient is determined based on the signal intensity of the NMR signal group difference. Weighting coefficients having different signs are used.
The magnetic resonance imaging method of the present invention further comprises a step of administering a contrast agent into the object to be examined, wherein the standard NMR signal group is measured before the contrast agent arrives at a predetermined portion of the object and the other NMR signal groups are measured while the contrast agent travels within the predetermined region of the object.
Each of the NMR signal groups is capable of producing a two-dimensional image or three-dimensional image. The obtained NMR signal groups may be subjected to subtraction and cumulative addition or weighted addition after each is reconstructed into image data or may be subjected to a subtraction as measured complex signals.
In an other aspect, the present invention provides an MRI apparatus comprising means for applying a static magnetic field, gradient magnetic field and RF magnetic field to an object to be examined by driving respective magnetic field generating units, means for detecting NMR signals emitted, from the object by NMR, means for performing an image reconstruction operation using the NMR signals detected by the detecting means, and means for displaying the obtained images, wherein the apparatus further comprises means for generating a plurality of NMR signal groups, on an individual image basis, for the same region of the object by controlling the magnetic field generating units, means for detecting and storing the plurality of NMR signal groups, means for generating a plurality of subtracted NMR signal groups by performing a subtraction operation between one standard NMR signal group and the other NMR signal groups, means for adding each of the plurality of subtracted NMR signal groups and means for displaying the result of the addition as an image.
The means for addition includes means for determining individual weighting coefficients for the plurality of subtracted NMR signal groups to be added.
The MRI apparatus of the present invention further comprises means for administering a contrast agent into the object to be examined, wherein the standard NMR signal group is measured before administration of the contrast agent and the other NMR signal groups are measured successively while the contrast agent travels within the same region of the object.